1. Field of the Invention
The present invention relates to a piston in a compressor, and in particular, it relates to an improvement of a sealing arrangement of a piston in a compressor such as used in an air conditioning system.
2. Description of the Related Art
In a compressor, a piston seal ring is provided in a ring groove of a piston to seal an annular gap between the piston and the cylinder bore. A typical piston seal ring is a discontinuous metal ring with a slit and can be used under tension. Recently, a continuous plastic seal ring has been developed, as disclosed in U.S. Pat. No. 4,351,227. Such a continuous plastic seal ring is referred to as a washer-like seal ring, and examples thereof are shown in FIGS. 8A and 8B of the attached drawings. The washer-like continuous seal ring 2 has an initial shape with a relatively thin width t.sub.0, like a washer used in various mechanical fields, as shown in FIG. 8A, and in use is deformed into a generally barrel-like shape, as shown in FIG. 8B. The initial seal ring 2 has an inner peripheral portion 2a, an outer peripheral portion 2b, and side surfaces 2c and 2d; the internal diameter of the inner peripheral portion 2a being smaller than an outer diameter of a piston 1 on which the seal ring 2 is to be attached Accordingly, the inner peripheral portion 2a of the seal ring 2 is radially expanded when the seal ring 2 is urged toward the piston 1, for example, in the direction of the arrow X, to fit the seal ring 2 in a ring groove of the piston 1.
When the seal ring 2 falls in the ring groove, it is further deformed, for example, by a suitable conical tool, in the reverse direction to the arrow X and the outer peripheral portion 2b is axially and inwardly collapsed relative to the inner peripheral portion 2a, so that one of the side surfaces 2d of the initial washer-like seal member 2 changes to an outer sealing surface of a barrel-like ring member 2 to be engaged with an inner surface of a cylinder bore in which the piston 1 is to be inserted. In this final shape, one of the side surfaces 2b (i.e., the outer peripheral portion 2b of the initial washer-like seal ring 2) has a thickness t.sub.1, which is generally equal to the width t.sub.0 of the initial seal ring 2, but the other side surface 2a (i.e., the inner peripheral portion 2a of the initial washer-like seal ring 2) has a reduced thickness t.sub.2 because the inner peripheral portion 2a has expanded. This seal ring 2 is advantageously used in a position, as shown in FIG. 8C, in which the thick side surface 2b is arranged on the side of the compression chamber A, i.e., on the side of the piston head la, to increase a sealing function of the compressed gas in the compression chamber A, and the thin side surface 2a is arranged on the opposite side. As shown in FIG. 8D, this barrel-like ring member 2 receives the high pressure of the compressed gas in the compression chamber A during a compression stroke of the compressor from the inner peripheral surface thereof, and is radially expanded into close contact with the inner surface of the cylinder bore 4. In this way, the barrel-like ring member 2 can hermetically slide in the inner surface of the cylinder bore 4 to provide a good sealing function, and be retained in the ring groove 15 by an abutting thereof against the side surface of the ring groove 15.
It will be understood that arrangement of the seal ring member 2 so that the thick side surface 2b faces toward the compression chamber A, as shown in FIG. 8C, is intended to minimize an outer clearance between the seal ring member 2 and the inner surface of the cylinder bore 4 (preferably to zero) and to provide an inner clearance between the seal ring 2 and the bottom of the ring groove 15 such that an escape of the compressed gas from the compression chamber A toward the unti-pistonhead area is prevented and is held in an inner gap between the inner surface of the seal ring 2 and the bottom of the ring groove 1 during the compression stroke of the compressor.
Nevertheless, the arrangement of the seal member 2 fitted in the ring groove 15 causes a problem in that the thickness t.sub.2 of the thin side surface 2a is reduced not only by the initial expansion of the inner peripheral portion 2a during insertion of the seal ring member 2 but also by a further expansion during the compression stroke of the compressor by the internally applied high pressure of the compressed gas, and thus the seal ring member 2 does not remain in the ring groove 15 and may be forced out into a gap between the inner surface of the cylinder bore 4 and the seal ring. 2.